DSP SRIO协议的逻辑层定义了操作协议和相应的包格式。DSP上SRIO支持的逻辑层业务(数据发送方法)主要是直接IO/DMA（Direct IO/ Direct Memory Access）和消息传递（Message Passing）。

Two Basic Modes of Operation:
– DirectIO
       ? Read/write operations directed to specific memory address
              – Transmit device has knowledge of memory map of receiving device
       ? Functional units:
              – LSU (Load/Store Unit)
              – MAU (Memory Access Unit)
– Message Passing
       ? Mailbox and Letter designators
              – Transmit device does not need knowledge of memory map of receiving device
       ? Functional units:
              – TXU (Message Transmit Unit)
              – RXU (Message Receive Unit)
       ? DOORBELL（重要）

下文会对两种模式进行详解。